The basic transmit signal of an FMCW radar is a frequency ramp (also commonly known as a “chirp”). A chirp is a signal whose frequency varies linearly with time. For example, a mm-wave FMCW radar might transmit a chirp having a 4 GHz bandwidth that starts at 77 GHz and linearly ramps up to 81 GHz. The signal transmitted by the transmit (TX) antenna reflects off one or more objects and the reflected signal is received at one or more receive (RX) antennas.
An FMCW radar transmits a series of such equally spaced chirps in a unit called a frame. The corresponding signal received at the RX antennas is down-converted, digitized and then processed to obtain the range, velocity and angle of arrival of multiple objects in front of the radar.
Multiple-Input Multiple Output (MIMO) Radar is a technique to improve the angle estimation capability of FMCW radar. With a MIMO Radar, multiple TX antennas transmit to the same set of RX antennas. The signals emanating from the multiple TX antennas need to be orthogonal (i.e. should not interfere with one another). Some of the common methods of ensuring orthogonality are: Time Division Multiplexing (TDM-MIMO), Frequency Division Multiplexing (FDM-MIMO) and code-phase multiplexing. In TDM-MIMO the signals from the different TX antennas occupy different time slots. In FDM-MIMO the signals from multiple TX antennas are separated in frequency. In code-phase multiplexing, multiple TX antennas simultaneously transmit signals using different pseudo-random noise codes (PN-codes), which enable the signals to be separated at the receiver. FDM-MIMO imposes additional hardware complexity on both the transmit and the receive chain. Code-phase multiplexing results in degraded performance (due to de-spreading noise) and/or significantly increased computational requirements (for decoding intra-chirp code PN sequences).
Consequently, TDM-MIMO has advantages over the other methods for providing orthogonality. However, the TDM-MIMO mode of operation results in a reduction in the maximum unambiguous velocity that can be measured by the radar. Using conventional TDM_MIMO radar, any object moving with a velocity greater than the maximum unambiguous velocity (vmax) will have its velocity estimated incorrectly.